Еssay on sedimentary rocks

Sedimentary rocks are a type of rocks, formed by deposition of material in water environment, in the air or as a result of ice-flow activity on the land surface, in sea and ocean basins. Depending on the precipitation character sedimentary rocks are divided into clastic (formed mechanically by gravity and change of environmental dynamics), chemical (formed from water solutions achieving saturation concentration and as a result of exchange reactions) and biogenic (formed under the influence of organisms’ vital activities). Formation and allocation of sedimentary rocks are determined mainly by climatic and tectonic conditions.

Material sources for sedimentary rocks formation are: weathering products of magmatic, metamorphic and more ancient sediments; components dissolved in natural waters; atmosphere gases; organic remains of vegetable and animal origin; volcanics; cosmic material.

The grain size, form and combination determine the structure of sediments.

Sedimentary rocks form shelves, layers, lenses and other geological bodies of different form and size, beddings in the Earth crust normally-horizontally, sloping or as compound folds. Most of rocks are characterized by the stratified texture; the types of texture depend on the formation conditions, mainly on the environmental dynamics.

Classification of sedimentary rocks is based on their composition and genesis. Over 10 groups of sedimentary rocks are distinguished: clay (making about 50%), carbonate and sandstone (about 45% in sum), other types (clastic, ferrian, manganese, phosphatic, siliceous, salt, caustobioliths, etc) make less than 5%. There are also rocks of mixed composition.

Sedimentary rocks make about 10% of the Earth crust mass and cover 75% of its surface. Main part of them is concentrated on continents (500 mln km3) and continental slopes (190 mln km3), while there is 250 mln km3 of them on the ocean bottom. Over 75% of all extracted minerals (coal, oil, salts, iron ore, manganese, aluminium, gold, platinum, phosphorites, construction materials, etc.) are contained in sedimentary rocks.

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